Magnetic recording medium

ABSTRACT

A magnetic recording medium comprising a non-ferromagnetic substrate and a magnetic recording film formed on the substrate with an underlayer interposed therebetween, wherein the magnetic recording film comprises a plurality of magnetic layers and an interlayer made of a material having a B2 crystal structure or an interlayer made of Ru, disposed between the adjacent magnetic layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording medium suitable as a recording medium for a hard disc device.

2. Discussion of Background

In recent years, magnetic recording media have been developed for high density recording. As one of methods for reducing media noises, JP-A-63-146219 or IEEE TRANSACTIONS ON MAGNETICS, 26(5), 2700-2705(1990) discloses that a magnetic recording film is made to have a multilayer structure comprising a plurality of magnetic layers and an interlayer interposed between the adjacent magnetic layers to reduce the magnetic interaction between the adjacent magnetic layers. Further, as a means to obtain a high coercive force for high recording density, it has been reported to employ, as an underlayer, NiAl having B2 crystal structure (IEEE TRANSACTIONS ON MAGNETICS, 30(6), 3951-3953(1994), EP0704839A1).

In said JP-A-63-146219, in order to reduce noises, an Al₂O₃, SiO₂ or Cr film has been studied as the interlayer to be used for a magnetic recording medium having the above-mentioned magnetic recording film having a multilayer structure, but reduction of noises is still inadequate for an application in future wherein a GMR head will be used. In said EP0704839A1, NiAl has been studied as the underlayer, but no study has been made to let the magnetic recording film have a multilayer structure comprising a plurality of magnetic layers and an interlayer interposed between the adjacent magnetic layers to reduce the magnetic interaction.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magnetic recording medium having a medium noise substantially reduced and being capable of high density recording.

The present invention has been made to accomplish the above object and provides a magnetic recording medium comprising a non-ferromagnetic substrate and a magnetic recording film formed on the substrate with an underlayer interposed therebetween, wherein the magnetic recording film comprises a plurality of magnetic layers and an interlayer made of a material having a B2 crystal structure or an interlayer made of Ru, disposed between the adjacent magnetic layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the thickness of a NiAlRu interlayer or a Cr interlayer and S/Nt (signal to total noise ratio).

FIG. 2 is a graph showing the relation between the thickness of a NiAlRu interlayer or a Cr interlayer and Hc (coercive force).

FIG. 3 is a graph showing the relation between the thickness of a Ru interlayer or a Cr interlayer and S/Nt.

FIG. 4 is a graph showing the relation between the thickness of a Ru interlayer or a Cr interlayer and Hc.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The magnetic recording film in the present invention may take a structure comprising two magnetic layers and an interlayer made of a material having a B2 crystal structure, disposed therebetween, or a structure comprising three or more magnetic layers and an interlayer made of a material having the B2 structure, disposed between the respective adjacent magnetic layers. The material for the interlayer having the above B2 structure may, for example, be one member selected from the group consisting of NiAl, NiAlRu, NiAlNd, NiAlCr, NiAlPt and NiAlPd.

Otherwise, the magnetic recording film in the present invention may take a structure comprising two magnetic layers and an interlayer made of Ru disposed therebetween, or a structure comprising three or more magnetic layers and an interlayer made of Ru disposed between the respective adjacent magnetic layers.

The thickness of the interlayer made of the above-mentioned material having the B2 structure or the interlayer made of Ru, is preferably from 3 Å to 30 Å. If the thickness is less than 3 Å, the effect for reducing the noise tends to be inadequate. On the other hand, if it exceeds about 30 Å, the effect for reducing the noise tends to be hardly observed, and the coercive force tends to substantially decrease, or properties such as the resolution and PW50 (pulse width at 50% of pulse amplitude) required as a magnetic recording medium tend to deteriorate. From these viewpoints, the thickness of the interlayer is preferably from 3 Å to 30 Å, more preferably from 3 Å to 20 Å.

The non-ferromagnetic substrate in the present invention may be selected from substrates made of materials such as an aluminum alloy, glass and crystallized glass.

As the underlayer in the present invention, Cr or a Cr alloy may be employed. The Cr alloy may, for example, be CrMo, CrW, CrTi, CrV or CrMn.

Further, in the present invention, in the case of using a non-ferromagnetic substrate made of the above-mentioned material such as glass or crystallized glass, it is preferred that the magnetic recording medium has a structure wherein a seed layer made of a material having a B2 crystal structure, is formed on the non-ferromagnetic substrate, and the magnetic recording film is formed on the seed layer with the underlayer interposed therebetween.

The material having the B2 structure to be used as the above seed layer, may, for example, be NiAl, NiAlRu, NiAlNd, NiAlCr, NiAlPt or NiAlPd.

By providing such a seed layer on the non-ferromagnetic substrate made of e.g. glass or crystallized glass, the grain growth and the film formation of the Cr or Cr alloy underlayer thereon will be properly controlled, whereby the grain growth and the film formation of the magnetic recording film to be formed on the underlayer will properly be controlled.

The magnetic layers constituting the magnetic recording film in the present invention are preferably made of a Co alloy.

As the magnetic layers in the present invention, magnetic layers made of an alloy comprising Co as the main component and at least Cr and Pt, may be selected. Further, magnetic layers may be selected which are made of an alloy which further contains at least one member selected from the group consisting of Ta, Mo, W, Nb, V, Zr, B and Ti.

Particularly, as the magnetic layers, magnetic layers made of a CoCrPt alloy or a CoCrTaPt alloy, may be selected for use.

On the above-described magnetic recording film, a protective film and a lubricating film may further be provided to obtain the magnetic recording medium of the present invention.

As the protective film, a carbon type material, may, for example, be employed, and as the lubricating film, a perfluoropolyether type lubricant may, for example, be employed.

Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific examples.

EXAMPLE 1

A sputtering chamber was evacuated to a base pressure of 1×10⁻⁶ Torr, and then film forming was carried out as described below in an atmosphere of 5 mTorr at a substrate temperature of 220° C. by applying a substrate bias of −200V.

On a NiP/Al substrate having texture treatment applied thereto, a CrMo layer (thickness: 300 Å) was formed as an underlayer by a magnetron sputtering method using a target made of Cr₈₅Mo₁₅. Then, on the CrMo layer, a first magnetic layer made of Co₇₁Cr₁₇Ta₅Pt₇ (the amount of the respective components being atomic %) was formed in a thickness of 110 Å by a sputtering method. Then, a NiAlRu interlayer was formed thereon in a thickness within a range of from 5 Å to 70 Å by means of a Ni₄₅Al₅₀Ru₅ target. Further, a second magnetic layer made of Co₇₁Cr₁₇Ta₅Pt₇ was formed thereon in a thickness of 110 Å by a similar operation. Then, a carbon type protective film and a lubricating film were formed thereon to obtain a test sample of Example 1 of the magnetic recording medium of the present invention. The above NiAlRu interlayer was identified to be of B2 structure by the X-ray diffraction.

Further, a magnetic recording medium having the same structure as the test sample of Example 1 except that as the interlayer, an interlayer made of Cr was formed in a thickness within a range of from 5 Å to 70 Å, was prepared in the same manner and used as a test sample of Comparative Example 1.

FIG. 1 shows a change in the signal to the total noise ratio (S/Nt) when the thickness of the NiAlRu interlayer of Example 1, or the thickness of the Cr interlayer of Comparative Example 1, was changed within a range of from 0 to 50 Å. In the case of the NiAlRu interlayer, it is evident that S/Nt is improved as the thickness increases within a range of from 0 to 30 Å. Further, it is evident that S/Nt is improved as compared with the case of the Cr interlayer.

FIG. 2 shows the change in the coercive force (Hc) when the thickness of the NiAlRu interlayer of Example 1 or the thickness of the Cr interlayer of Comparative Example 1 was changed within a range of from 0 to 70 Å. It is evident that in the case of the NiAlRu interlayer, the decrease in the coercive force (Hc) is small as compared with the Cr interlayer.

When the magnetic recording film is made to have a structure of Co alloy magnetic layer/NiAlRu interlayer of B2 structure/Co alloy magnetic layer, and the thickness of the NiAlRu interlayer of B2 structure is properly selected within a range of from about 3 Å to about 30 Å, more preferably at most 20 Å, a remarkable improvement in S/Nt can be attained, and the value is large as compared with the case of the Cr interlayer. By this selection, the coercive force (Hc) decreases, but the degree of the decrease is small and will not be a trouble in accomplishing a high density recording. In the case of the Cr interlayer, the improvement in S/Nt is small, and the decrease in the coercive force (Hc) is substantial.

The read/write performance with the thickness of the NiAlRu interlayer of 15 Å where the improvement in S/Nt is remarkable, is shown in Table 1. It is evident that with the thickness of the NiAlRu interlayer of 15 Å, not only N media (media noise), S/Nt and S/Nm (signal to media noise ratio), but also the resolution and PW50 (pulse width at 50% of pulse amplitude), are excellent as compared with the case where the Cr interlayer was employed.

TABLE 1 Resolution PW50 O/W N media S/Nt S/Nm (%) (nsec) (−dB) (mV²) (dB) (dB) Example 1 74.13 18.41 35.6 2.53 23.6 28.6 Comparative 72.46 18.85 36.8 4.17 20.1 26.3 Example 1

EXAMPLE 2

A sputtering chamber was evacuated to a base pressure of 1×10⁻⁶ Torr, and then film forming was carried out as described below in an Ar atmosphere of 5 mTorr at a substrate temperature of 270° C. without applying a substrate bias.

On a non-ferromagnetic substrate made of aluminosilicate glass, a NiAl seed layer (thickness: 500 Å) was firstly formed by a magnetron sputtering method using a target made of Ni₅₀Al₅₀ having B2 structure, and then a Cr layer (thickness: 300 Å) was formed as an underlayer.

Then, on the Cr layer, a first magnetic layer made of Co₇₁Cr₁₇Ta₄Pt₈ (the amounts of the respective components being atomic %), was formed in a thickness of 110 Å by a sputtering method. Then, a Ru interlayer was formed thereon in a thickness within a range of 5 Å to 50 Å by means of a Ru target. Further, a second magnetic layer made of Co₇₁Cr₁₇Ta₄Pt₈ was formed thereon in a thickness of 110 Å by a similar operation. Then, a carbon type protective layer and a lubricating layer were formed thereon to obtain a test sample of Example 2 of the magnetic recording medium of the present invention. By the X-ray diffraction, the above NiAl seed layer was identified to be NiAl having B2 structure, having a composition of Ni₅₀Al₅₀, and the above Ru interlayer was identified to be of a hexagonal closest packed structure.

Further, a magnetic recording medium having the same structure as the test sample of Example 2 except that as the interlayer, an interlayer made of Cr was formed in a thickness within a range of from 5 Å to 50 Å, was prepared in the same manner and used as a test sample of Comparative Example 2.

FIG. 3 shows the change in the signal to the total noise ratio (S/Nt) when the thickness of the Ru interlayer of Example 2 or the thickness of the Cr interlayer of Comparative Example 2, was changed within a range of from 0 to 50 Å. It is evident that in the case where the Ru interlayer was used, S/Nt is improved as the thickness increases within a range of from 0 to 30 Å. Further, it is evident that S/Nt is improved also as compared with the case where the Cr interlayer was used.

FIG. 4 shows the change in the coercive force (Hc) when the thickness of the Ru interlayer of Example 2 or the thickness of the Cr interlayer of Comparative Example 2, was changed within a range of from 0 to 50 Å. It is evident that in the case of the Ru interlayer, the decrease in the coercive force is small as compared with the case of the Cr interlayer.

When the magnetic recording film is made to have a structure of Co alloy magnetic layer/Ru interlayer/Co alloy magnetic layer, and the thickness of the Ru interlayer is properly selected within a range of from about 3 Å to about 30 Å, preferably at most about 20 Å, remarkable improvement in S/Nt can be attained, and the value is large as compared with the case of the Cr interlayer. By this selection, the coercive force (Hc) will decrease, but the degree of the decrease is small and will not be any trouble in accomplishing a high density recording. In the case of the Cr interlayer, the improvement in S/Nt is small, and the decrease in the coercive force (Hc) is substantial.

The read/write performance with the thickness of the Ru interlayer of 10 Å where the improvement in S/Nt is remarkable, is shown in Table 2. It is evident that with the thickness of the Ru interlayer of 10 Å, not only N media (media noise), S/Nt and S/Nm (signal to media noise ratio), but also the resolution and PW50 (pulse width at 50% of pulse amplitude), are excellent as compared with the case where the Cr interlayer was used.

TABLE 2 Resolution PW50 O/W N media S/Nt S/Nm (%) (nsec) (−dB) (mV²) (dB) (dB) Example 2 73.13 24.57 38.5 23.86 17.4 19.3 Comparative 70.05 26.10 41.5 24.50 15.8 17.7 Example 2

EXAMPLE 3

A sputtering chamber was evacuated to a base pressure of 1×10⁻⁶ Torr, and then film forming was carried out as described below in an Ar atmosphere of 5 mTorr at a substrate temperature of 270° C. without applying a substrate bias.

On a non-ferromagnetic substrate made of aluminosilicate glass, a NiAl seed layer (thickness: 500 Å) was firstly formed by a magnetron sputtering method using a target made of Ni₅₀Al₅₀ having B2 structure, and then a Cr₈₅Mo₁₅ layer (thickness: 100 Å) was formed as an underlayer.

Then, on the CrMo layer, a first magnetic layer made of Co₆₈Cr₂₀Ta₂Pt₁₀ (the amounts of the respective components being atomic %), was formed in a thickness of 110 Å by a sputtering method. Then, a Ru interlayer was formed thereon in a thickness of 10 Å (i.e. the thickness of the Ru interlayer in which the maximum value of S/Nt was obtained in Example 2) by means of a Ru target. Further, a second magnetic layer made of Co₆₈Cr₂₀Ta₂Pt₁₀ was formed thereon in a thickness of 110 Å by a similar operation. Then, a carbon type protective layer and a lubricating layer were formed thereon to obtain a test sample of Example 3 of the magnetic recording medium of the present invention. By the X-ray diffraction, the above NiAl seed layer was identified to be NiAl having B2 structure, having a composition of Ni₅₀Al₅₀, and the above Ru interlayer was identified to be of a hexagonal closest packed structure.

Further, a magnetic recording medium having the same structures of the substrate, the seed layer and the underlayer as the sample of Example 3 except that as the magnetic recording film on the underlayer, a magnetic recording film (a single layer film) composed of a single magnetic layer made of Co₆₈Cr₂₀Ta₂Pt₁₀ was formed in a thickness of 220 Å, was prepared in the same manner and used as a test sample of Comparative Example 3.

In a case where NiAl having B2 structure is employed as the seed layer, when the magnetic recording film is made to have a film structure of Co alloy magnetic layer/Ru interlayer/Co alloy magnetic layer, and the thickness of the Ru interlayer is properly selected within a range of from about 3 Å to about 30 Å, preferably at most about 20 Å, remarkable improvement in S/Nt can be attained. By this selection, the coercive force (Hc) will decrease, but the degree of the decrease is small and will not be any trouble in accomplishing a high density recording.

Hc (Oe) and the read/write performance with the thickness of the Ru interlayer of 10 Å where the improvement in S/Nt is remarkable, are shown in Table 3. It is evident that with the thickness of the Ru interlayer of 10 Å, not only N media (media noise), S/Nt and S/Nm (signal to media noise ratio), but also the resolution and PW50 (pulse width at 50% of pulse amplitude), are excellent with the magnetic recording medium having a magnetic recording film of a double layer structure of the present invention, as compared with the case where a magnetic recording film (single layer film) made of single magnetic layer is used.

TABLE 3 Resolu- PW50 O/W N media S/Nt S/Nm Hc tion (%) (nsec) (−dB) (mV²) (dB) (dB) (Oe) Example 3 62.10 18.95 37.05 16.72 23.06 26.71 2500 Compara- 61.20 19.10 35.86 44.90 20.49 22.10 2600 tive Example 3

The magnetic recording medium of the present invention has a feature that the noise is remarkably reduced with the structure comprising a non-ferromagnetic substrate and a magnetic recording film formed on the substrate with an underlayer interposed therebetween, wherein the magnetic recording film comprises a plurality of magnetic layers and an interlayer made of a material having a B2 crystal structure or an interlayer made of Ru, disposed between the adjacent magnetic layers.

Further, especially when a non-ferromagnetic substrate made of glass or crystallized glass is employed, the magnetic recording medium of the present invention has an excellent feature that the noise can be remarkably reduced by providing a seed layer made of a material having a B2 crystal structure on the substrate and providing the above-mentioned magnetic recording film of a double layer structure thereon with an underlayer made of Cr or a Cr alloy interposed therebetween.

Furthermore, the magnetic recording medium of the present invention has excellent characteristics also with respect to the resolution and PW50. 

What is claimed is:
 1. A magnetic recording medium comprising a non-ferromagnetic substrate and a magnetic recording film formed on the substrate with an underlayer interposed therebetween, wherein the magnetic recording film comprises a plurality of magnetic layers and an interlayer made of a material having a B2 crystal structure disposed between the adjacent magnetic layers.
 2. The magnetic recording medium according to claim 1, wherein the material for the interlayer having the B2 structure is one member selected from the group consisting of NiAl, NiAlRu, NiAlNd, NiAlCr, NiAlPt and NiAlPd.
 3. The magnetic recording medium according to claim 1, wherein the thickness of the interlayer made of a material having the B2 structure is from 3 Å to 30 Å.
 4. The magnetic recording medium according to claim 1, wherein the thickness of the interlayer made of a material having the B2 structure is from 3 Å to 20 Å.
 5. The magnetic recording medium according to claim 1, wherein the underlayer is made of Cr or a Cr alloy.
 6. The magnetic recording medium according to claim 1, wherein a seed layer made of a material having a B2 crystal structure, is formed on the non-ferromagnetic substrate, and the magnetic recording film is formed on the seed layer with the underlayer interposed therebetween.
 7. The magnetic recording medium according to claim 6, wherein the material for the seed layer having the B2 structure is one member selected from the group consisting of NiAl, NiAlRu, NiAlNd, NiAlCr, NiAlPt and NiAlPd.
 8. The magnetic recording medium according to claim 1, wherein the magnetic layers are made of a Co alloy.
 9. The magnetic recording medium according to claim 1, wherein the magnetic layers are made of a CoCrPt alloy or a CoCrTaPt alloy.
 10. A magnetic recording medium comprising a non-ferromagnetic substrate and a magnetic recording film formed on the substrate with an underlayer interposed therebetween, wherein the magnetic recording film comprises a plurality of magnetic layers and an interlayer made of Ru, disposed between the adjacent magnetic layers, wherein the thickness of the interlayer made of Ru is from 3 Å to less than 10 Å.
 11. The magnetic recording medium according to claim 10, wherein the underlayer is made of Cr or a Cr alloy.
 12. The magnetic recording medium according to claim 10, wherein a seed layer made of a material having a B2 crystal structure, is formed on the non-ferromagnetic substrate, and the magnetic recording film is formed on the seed layer with the underlayer interposed therebetween.
 13. The magnetic recording medium according to claim 12, wherein the material for the seed layer having the B2 structure is one member selected from the group consisting of NiAl, NiAlRu, NiAlNd, NiAlCr, NiAlPt and NiAlPd.
 14. The magnetic recording medium according to claim 10, wherein the magnetic layers are made of a Co alloy.
 15. The magnetic recording medium according to claim 10, wherein the magnetic layers are made of a CoCrPt alloy or a CoCrTaPt alloy. 